This invention relates to methods of producing sound recordings and to the sound recordings produced thereby, and is particularly concerned with stereo sound production methods.
It is possible to give a listener the impression that there is a sound source, referred to as a virtual sound source, at a given position in space provided that the sound pressures that are reproduced at the listener's ears are the same as the sound pressures that would have been produced at the listener's ears by a real source at the desired position of the virtual source. This attempt to deceive the human hearing can be implemented by using either headphones or loudspeakers. Both methods have their advantages and drawbacks.
Using headphones, no processing of the desired signals is necessary irrespective of the acoustic environment in which they are used. However, headphone reproduction of binaural material often suffers from ‘in-the-head’ localisation of certain sound sources, and poor localisation of frontal and rear sources. It is generally very difficult to give the listener the impression that the virtual sound source is truly external, i.e. ‘outside the head’.
Using loudspeakers, it is not difficult to make the virtual sound source appear to be truly external. However, it is necessary to use relatively sophisticated digital signal processing in order to obtain the desired effect, and the perceived quality of the virtual source depends on both the properties (characteristics) of the loudspeakers and to some extent the acoustic environment.
Using two loudspeakers, two desired signals can be reproduced with great accuracy at two points in space. When these two points are chosen to coincide with the positions of the ears of a listener, it is possible to provide very convincing sound images for that listener. This method has been implemented by a number of different systems which have all used widely spaced loudspeaker arrangements spanning typically 60 degrees as seen by the listener. A fundamental problem that one faces when using such a loudspeaker arrangement is that convincing virtual images are only experienced within a very confined spatial region or ‘bubble’ surrounding the listener's head. If the head moves more than a few centimeters to the side, the illusion created by the virtual source image breaks down completely. Thus, virtual source imaging using two widely spaced loudspeakers is not very robust with respect to head movement.
We have discovered, somewhat surprisingly, that a virtual sound source imaging form of sound reproduction system using two closely spaced loudspeakers can be extremely robust with respect to head movement. The size of the ‘bubble’ around the listener's head is increased significantly without any noticeable reduction in performance. In addition, the close loudspeaker arrangement also makes it possible to include the two loudspeakers in a single cabinet.
From time to time herein, the present invention is conveniently referred to as a ‘stereo dipole’, although the sound field it produces is an approximation to the sound field that would be produced by a combination of point monopole and point dipole sources.